1. Field of the Invention
The invention relates to a corona-shielding arrangement for the stator winding of an electric machine, comprising a semiconductive textile coronashielding strip wound around the major or main insulation of the slot portion and end yoke portion of each bar of the stator winding.
2. Discussion of Background
The stator winding conductors of rotating electric machines, which are already provided over their entire length with the major or main insulation, are provided in the slot portion with coil-side corona shielding. Nowadays, the latter generally consists of a semiconductive nonwoven or fabric already applied during the insulating operation. The coil-side corona shielding must be led out so far beyond the laminate stack length that no discharges can occur even given small distances with respect to the pressure plates and pressure fingers of the laminate stack.
In machines having relatively high nominal voltages (5 kvolts and higher), overhang corona shielding adjoins the coil-side corona shielding. This overhang corona shielding ensures that the decrease in potential along the conductor takes place towards the grounded laminate stack without the possibility of creeping or glow discharges. The overhang corona shielding must be calculated and designed not only for the operating stress, but also, above all, for the test voltages to be applied for monitoring the production quality. A specific insulation length is required for the voltage reduction by means of such overhang corona shielding. In the case of insulations which have an adequate dielectric strength only in the slot portion, the straight overhang projection must therefore be sufficiently long to control the voltages. Continuous insulations, by contrast, can be designed such that the corona-shielding arrangement can also be applied in the involute directly adjoining the slot portion. This permits a desired relatively short overhang projection, but also requires the full insulation thickness up to the end of the corona shielding. Either surface coatings in the form of paint finishes or semiconductive strips having a specific conductivity value are applied for the purpose of voltage control (cf. the series "Herstellung der Wicklungen elektrischer Maschinen" ["Production of the Windings of Electric Machines"], Springer-Verlag, Vienna - N.Y., 1973, pages 156 to 158). If it is desired in this case to achieve as linear as possible a voltage characteristic over the length of the winding overhang conductor, problems arise in the use of semiconductive paint finishes, because such paint finishes essentially have only a surface conductivity. The relevant literature throws no light on how the voltage characteristic can be matched/optimized in the case of the use of semiconductive strips.